代理模式 - 简书

静态代理

场景1 :
记录登陆时间,注册时间:

public class UserController {
    private MetricsCollector metricsCollector;
  
    public UserVo login(String telephone,String passwd){
        long startTimeStamp = System.currentTimeMillis();
        // log in logic
        
        long endTimeStamp = System.currentTimeMillis();
        long loginTime = endTimeStamp - startTimeStamp;

        metricsCollector.recordRequest(loginTime);
    }
    
    public UserVo register(String telephone, String passwd) {
        long startTimeStamp = System.currentTimeMillis();
        // register  logic

        long endTimeStamp = System.currentTimeMillis();
        long loginTime = endTimeStamp - startTimeStamp;

        metricsCollector.recordRequest(loginTime);
        return userVo;
    }
}

显然新加入记录时间功能加入了原有的逻辑中,对之后的拓展加大了阻碍.

通过定义接口:

public interface IUserController {
    public UserVo login(String telephone,String passwd)
    public UserVo register(String telephone,String passwd)
}

让UserController , UserControllerProxy 都实现IUserController.

public class UserController  implements  IUserController{
   @Override
   public UserVo login(String telephone,String passwd){
        // log in logic
        return userVO;
    }
    @Override
    public UserVo register(String telephone, String passwd) {
        // register logic
        return userVo;
    }
}

public class UserControllerProxy  implements IUserController{

    private  UserController controller;
    private  MetricsCollector metricsCollector;

    public UserControllerProxy(UserController userController){
        this.controller = userController;
        this.metricsCollector = new MetricsCollector();
    }
    @Override
    public UserVo login(String telephone, String passwd) {
        long startTimeStamp = System.currentTimeMillis();

        //origin logic
        UseVo useVo = this.controller.login(telephone,passwd);

        long endTimeStamp = System.currentTimeMillis();
        long loginTime = endTimeStamp - startTimeStamp;
        this.metricsCollector.recordRequest(loginTime);
        return null;
    }

    @Override
    public UserVo register(String telephone, String passwd) {
        long startTimeStamp = System.currentTimeMillis();

        //origin logic
        UseVo useVo = this.controller.register(telephone,passwd);

        long endTimeStamp = System.currentTimeMillis();
        long registerTime = endTimeStamp - startTimeStamp;
        this.metricsCollector.recordRequest(registerTime);
        return null;
    }
}

在调用的时候通过代理调用:

IUserController userController = new UserControllerProxy(new UserController());

达到的尽量少的改动原来的代码目的.

注: 在这里我们给加多了一个接口 IUserController,但是如果原来的UserController 是一个第三方的类,我们不能修改. 那么我们就要通过继承并且重写相应的方法来实现代理了

public class UserControllerProxy1  extends UserController{
    private  MetricsCollector metricsCollector;
    public UserControllerProxy1(){
        this.metricsCollector = new MetricsCollector();
    }

    @Override
    public UserVo login(String telephone, String passwd) {
        long startTimeStamp = System.currentTimeMillis();

        //origin logic
        UseVo useVo = super.login(telephone,passwd);

        long endTimeStamp = System.currentTimeMillis();
        long loginTime = endTimeStamp - startTimeStamp;
        this.metricsCollector.recordRequest(loginTime);
        return useVo;
    }
    @Override
    public UserVo register(String telephone, String passwd) {
        long startTimeStamp = System.currentTimeMillis();

        //origin register
        UseVo useVo = super.register(telephone,passwd);

        long endTimeStamp = System.currentTimeMillis();
        long registerTime = endTimeStamp - startTimeStamp;
        this.metricsCollector.recordRequest(registerTime);
        return useVo;
    }
}

如果附加功能只有一个还好,要是有多个就需要些很多代理类,并且实现一些方法这样子以来有增加了成本.

应用的场景: 监控,统计,限流,事物日志.

JAVA 的动态代理 Spring 底层AOP 就是通过动态代理来实现的.

public class MetricsCollectorProxy {

    private  MetricsCollector metricsCollector;

    public MetricsCollectorProxy(){
        this.metricsCollector = new MetricsCollector();
    }

    public Object createProxy(Object proxyObject){
        Class<?>[] interfaces =  proxyObject.getClass().getInterfaces();
        DynamicProxyHandler handler = new DynamicProxyHandler(proxyObject);
        
        return Proxy.newProxyInstance(proxyObject.getClass().getClassLoader(),interfaces,handler);
    }
    
    class DynamicProxyHandler implements InvocationHandler{
        private Object proxyObject;
        
        DynamicProxyHandler(Object proxyObject){
            this.proxyObject = proxyObject;
        }
        @Override
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            long startTimeStamp = System.currentTimeMillis();
            // register
            method.invoke(proxyObject,args);

            long endTimeStamp = System.currentTimeMillis();
            long loginTime = endTimeStamp - startTimeStamp;

            metricsCollector.recordRequest(loginTime);
            return null;
        }
    }